FIG. 1 shows a block diagram of a balun 10, which is a circuit that performs a signal conversion between a single-ended (unbalanced) port 12 and two differential (balanced) ports 14, 16. In some implementations, the single-ended port 12 is a signal input and the two differential ports 14 16 are signal outputs. In other implementations, the single-ended port 12 is a signal output and the two differential ports 14, 16 are signal inputs. The single-ended or unbalanced port of a balun is a signal port that receives a signal over a cable (e.g., a coaxial cable) that uses a single conductor to transmit the signal, which typically is referenced to the device ground potential. The differential or balanced ports of a balun are signal ports that receive a signal over a cable (e.g., a twisted-pair cable) that has two identical conductors for carrying voltages that are opposite in polarity but equal in magnitude with respect to ground. The balun 10 typically is used to connect components with unbalanced inputs or outputs (e.g., an antenna) to signal lines and components with balanced inputs or outputs (e.g., a differential amplifier), which typically are characterized by superior immunity to electromagnetic interference, power supply noise and ground noise relative to unbalanced components. The balun 10 also typically passively transforms the impedance between the single-ended port 12 and the two differential ports 14, 16.
Baluns commonly are implemented by transformers that have a first winding connected to the single-ended port and a second winding connected to the two differential ports. Recently, baluns have been implemented by bulk acoustic wave (BAW) resonators.
Known BAW resonators include one or more piezoelectric layers disposed between two electrodes. For example, thin film bulk acoustic wave resonators (FBARs) typically include a single piezoelectric layer between two electrodes. Stacked thin film bulk acoustic wave resonators (SBARs), on the other hand, typically include two or more piezoelectric layers disposed between top and bottom electrode layers and separated from each other by one or more intervening electrodes. Some acoustic resonator devices include multiple acoustic resonators that are isolated from one another by respective decouplers, which are formed by one or more dielectric layers that provide only weak acoustic coupling between the acoustic resonators. A BAW resonator has a resonant frequency that is determined by the thickness of the piezoelectric layer and by the thicknesses and the materials used for the other layers. A BAW resonator typically is acoustically isolated from the supporting substrate by an acoustic isolator, which may include a cavity formed under a membrane supporting a BAW resonator or an acoustic mirror that includes of a stack of layers alternately formed of high and low acoustic impedance materials and having respective thicknesses of approximately one-quarter of the wavelength corresponding to the target resonant frequency of the device. A BAW resonator that is disposed on an acoustic mirror often is referred to as a solidly mounted resonator (SMR).
FIG. 2 shows an example of a thin film bulk acoustic wave resonator (FBAR) 18 that includes a single piezoelectric layer 20 between two electrodes 22, 24. The FBAR 18 is acoustically isolated from the underlying substrate by an acoustic isolator 26. In operation, a time-varying electrical signal at the resonant frequency of the FBAR 18 is applied across the electrodes 22, 24. The applied electrical signal induces acoustic vibrations parallel to a propagation axis 28, which is normal to the electrodes 22, 24.
FIG. 3 shows an example of a prior art balun 30 that is implemented using FBARs. The balun 30 includes a dielectric layer 32 separating a first FBAR 34 and a second FBAR 36. The first FBAR 34 consists of the overlapping regions of a piezoelectric layer 38, a top electrode 40, and a bottom electrode 42. The second FBAR 36 consists of the overlapping regions of a piezoelectric layer 44, a top electrode 46, and a bottom electrode 48. The active area of the balun 30 corresponds to the overlapping regions of the first and second FBARs 34, 36 (shown by the dashed box). In this example, the bottom electrode 42 of the first FBAR 34 is connected to the single-ended port 12, the top electrode 40 of the first FBAR 34 is connected to the device ground, the bottom electrode 48 of the second FBAR 36 is connected to the first differential port 14, and the top electrode 46 of the second FBAR 36 is connected to the second differential port 16. In this design, the material compositions and thicknesses of the constituent layers of the first and second FBARs 34, 36 are the same. Consequently, the balun 30 produces a 1:1 impedance transformation between the single-ended port 12 and the differential ports 14, 16.
FIG. 4 shows another example of a prior art balun 50 that includes a first SBAR 52 and a second SBAR 54. The first SBAR 52 is formed from a dielectric layer 55 between a first FBAR 56 and a second FBAR 58. The second SBAR 54 is formed from a dielectric layer 59 between a third FBAR 60 and a fourth FBAR 62. The bottom electrode of the first FBAR 56 is connected to the single-ended port 12 and to the top electrode of the third FBAR 60. The top electrode of the first FBAR 56, the bottom electrode of the second FBAR 58, the bottom electrode of the third FBAR 60, and the bottom electrode of the fourth FBAR 62 all are connected to the device ground. The top electrode of the second FBAR 58 is connected to the first differential port 14 and the top electrode of the fourth FBAR 62 is connected to the second differential port 16. In this design, the first, second, third, and fourth FBARs 56, 58, 60, 62 are the same. Consequently, the balun 50 produces a 1:4 impedance transformation from the single-ended port 12 to the differential ports 14, 16.
Although baluns that provide 1:1 and 1:4 impedance transformations have a wide variety of useful applications, some applications would benefit from baluns that may be tailored to provide customized impedance transformations that are different from 1:1 and 1:4. What are needed are impedance transforming BAW baluns that are capable of flexibly providing a wide variety of different impedance transformations between the balanced ports and the unbalanced ports without increasing losses within the devices.